1. Field of the Invention
The present invention relates to a combustor for use in machines and equipments that require a feed of a high temperature gaseous medium to, for example, a gas turbine engine or a boiler.
2. Description of the Related Art
In the field of gas turbine engines, for due consideration to the environmental protection, severe standards have been stipulated on the composition of exhaust gases emitted as a result of combustion taking place therein and, hence, reduction of hazardous substances such as, nitrogen oxides (hereinafter referred to as NOx) is required. On the other hand, in heavy duty gas turbines and aircraft engines, the pressure ratio is increasingly set to be high to accommodate demands for low fuel consumption and high output capacity. In consistency therewith, a high temperature, high pressure is employed at an inlet to the combustor. High temperature at the inlet to the combustor leads to a casual increase of the combustion temperature, which brings about a rising concern that NOx in the exhaust gases may eventually increase.
In view of the above, a complex combustion system has come to be suggested, in which a lean pre-mix combustion system effective to reduce the NOx emission level and a diffusive combustion system excellent in ignition performance and flame holding performance are combined together (See, for example, the Patent Documents 1 and 2 listed below). The lean pre-mix combustion system referred to above has such an advantage that since an air/fuel mixture, prepared by pre-mixing air and fuel to have a uniform fuel concentration, is burned, there is no combustion region, at which the flame temperature is locally high, and since the fuel is leaned, the flame temperature can be totally lowered and the amount of NOx emitted can be effectively reduced. However, the lean pre-mix combustion system has such a problem that since large amounts of air and fuel are uniformly mixed, the local fuel concentration in the combustion region tends to become lean, accompanied by lowering of the combustion stability, that is, the flame holding capability particularly at a low load condition. On the other hand, the diffusive combustion system referred to above has such an advantage that since fuel and air are burned while being diffused and mixed, the blow off will hardly occur even at a low load condition while the flame holding capability is excellent. Accordingly, the complex combustion system referred to above is of a type, in which at starting and also at a low load condition the diffusion combustion is utilized to secure the combustion stability and, on the other hand, at a high load condition the pre-mix gas combustion is utilized to reduce the NOx emission level.
A combustor in accordance with related art utilizing the complex combustion system makes use of, for example, as shown in FIG. 6, a burner unit 85 including a diffusive fuel burner (pilot burner) 84, which is operable to inject a diffusive fuel into the combustion chamber and is arranged at a top portion of a combustion liner 81 of the combustor 80, and a pre-mix fuel burner (main burner) 82 for injecting a pre-mix gas into the combustion chamber so as to surround the outside of the injected diffusive fuel. The pilot burner 84 employed therein is in the form of a swirling type burner including an air injecting port 84b for injecting a stream of air A, which has become a swirling flow through a swirler 86, around a fuel injecting port 84a at the center thereof.    [Patent Document 1] JP Laid-open Patent Publication No. H08-28871    [Patent Document 2] JP Laid-open Patent Publication No. H08-210641